For the sauce drain the cherries. Stir cornflour into 2 tablespoons of cherry juice until smooth. Boil up the rest of the juice, sugar and lemon juice. Stir in the cornflour. Bring to the boil again briefly and allow to cool. For the mousse, melt white chocolate on a hot water bath.
Chop the dark chocolate. Whip the cream, vanilla sugar and cream stiffener until stiff. Stir into the melted chocolate. Fold in half of the chopped chocolate. Pour into a bowl and chill. Just before serving, decorate with cream tuffs, some cherries from the sauce and the rest of the chopped chocolate